New Lessons from the HI Size-Mass Relation of Galaxies

TL;DR

This study confirms a very tight and consistent relation between the size and mass of neutral hydrogen in a large sample of nearby galaxies, providing new constraints for galaxy formation models.

Contribution

It provides a comprehensive analysis of the HI size-mass relation across diverse galaxy types, revealing its tightness and implications for galaxy evolution.

Findings

The HI size-mass relation has a scatter of only 14%.

The relation is consistent across galaxy luminosities and types.

Different gas accretion histories are indicated by profile variations.

Abstract

We revisit the HI size-mass (D$_{\rm HI}$-M$_{\rm HI}$) relation of galaxies with a sample of more than 500 nearby galaxies covering over five orders of magnitude in HI mass and more than ten $B$-band magnitudes. The relation is remarkably tight with a scatter $\sigma \sim$0.06 dex, or 14%. The scatter does not change as a function of galaxy luminosity, HI richness or morphological type. The relation is linked to the fact that dwarf and spiral galaxies have a homogenous radial profile of HI surface density in the outer regions when the radius is normalised by D$_{\rm HI}$. The early-type disk galaxies typically have shallower HI radial profiles, indicating a different gas accretion history. We argue that the process of atomic-to-molecular gas conversion or star formation cannot explain the tightness of the D$_{\rm HI}$-M$_{\rm HI}$ relation. This simple relation puts strong constraints on simulation models for galaxy formation.